Acetylcholine receptor channels activated by a single agonist molecule

The neuromuscular acetylcholine receptor (AChR) is an allosteric protein that alternatively adopts inactive versus active conformations (R↔R*). The R* shape has a higher agonist affinity and ionic conductance than R. To understand how agonists trigger this gating isomerization, we examined single-channel currents from adult mouse muscle AChRs that isomerize normally without agonists but have only a single site able to use agonist binding energy to motivate gating. We estimated the monoliganded gating equilibrium constant E₁ and the energy change associated with the R versus R* change in affinity for agonists. AChRs with only one operational binding site gave rise to a single population of currents, indicating that the two transmitter binding sites have approximately the same affinity for the transmitter ACh. The results indicated that E₁ ≈ 4.3 x 10^-3 with ACh, and ≈ 1.7 x 10^-4 with the partial-agonist choline. From these values and the diliganded gating equilibrium constants, we estimate that the unliganded AChR gating constant is E ₀ ≈ 6.5 x 10^-7. Gating changes the stability of the ligand-protein complex by ∼5.2 kcal/mol for ACh and ∼3.3 kcal/mol for choline.